Fix compiler warnings in the pystrehex module (GH-13730)

Author: pablogsal

Python/pystrhex.c

@@ -11,11 +11,11 @@ static PyObject *_Py_strhex_impl(const char* argbuf, const Py_ssize_t arglen,
     PyObject *retval;
     Py_UCS1* retbuf;
     Py_ssize_t i, j, resultlen = 0;
-    Py_UCS1 sep_char;
+    Py_UCS1 sep_char = 0;
     unsigned int abs_bytes_per_sep;
 
     if (sep) {
-        Py_ssize_t seplen = PyObject_Length(sep);
+        Py_ssize_t seplen = PyObject_Length((PyObject*)sep);
         if (seplen < 0) {
             return NULL;
         }